Speaker cones and parts having good acoustical properties made from aramid papers are known. Aramid papers for speaker cones generally combine crystallized p-aramid fibers and amorphous m-aramid fibrids; the fibrids act as a binder for the p-aramid fibers by softening and bonding the fibers when the formed sheets are subjected to high pressure and temperature.
Aramid papers typically have coloring similar to that of the base fiber. Generally, papers made from natural colored p-aramid fiber, such as the fiber known under the DuPont trademark KEVLAR, range in color from golden to cream-beige. Papers made from natural colored m-aramid fiber, such as the fiber known under the DuPont trademark NOMEX, range in color from off-white to light beige. Coloring, other than the natural coloring of aramid papers, has been achieved in the past by using pigmented p-aramid fibers in the make-up of the sheets or by printing a color layer on the surface of the formed sheet.
Often when using aramid papers in many applications or for articles molded from aramid papers, colors other than the natural color of the fibers or fibrids are desired for aesthetic reasons. Molders of various parts, particularly speaker cones and molded parts which are visible in use, normally require materials having some variety of colors, particularly dark shades and black.
Typically, molders use a printing step to impart surface color to the nonwoven aramid sheets before they are shaped into molded parts. Experience has shown that when printed aramid paper is molded, surface color defects occur causing the surface of the molded part to appear to have cracks. Such "cracks" make the appearance of a speaker made with such cones unsatisfactory. These unappealing surface defects arise from a shifting of the surface components of the sheet during the molding process. During molding heat and shear are applied to the sheet to further densify and form the sheet into a specified shape. While under this shearing force, printed fibrids and/or fibers may shift exposing unprinted fibrids and/or fibers from below the surface of the sheet. Areas where the unprinted fibrids or fibers become visible appear to the eye to be cracks in the colored surface. In reality these "cracks" are just unprinted streaks in the paper surface, and although these "cracks" have no effect on the function or performance of the molded parts or speaker cones, they are aesthetically objectionable.
Fibrids filled with activated carbon have been taught by Bair in U.S. Pat. No. 5,482,773, and the use of fillers in the making of fibrids was taught by Morgan in U.S. Pat. No. 2,999,788. The presence of fillers in fibrids tend to alter the mechanical properties of the fibrid. The fibrids of Bair were used as an absorber and had high content (40 to 85 weight %) of activated carbon. Both the fibrids taught by Bair and sheets made from these fibrids were of only fair mechanical quality as a result of the high concentration of carbon filler.
The present invention provides colored fibrids that can be used in the making of colored aramid sheets and molded sheet products. The fibrids of the present invention are colored in such a way that they, and sheet products made from them, are not compromised in mechanical properties. This is of particular importance in considering the performance of a sheet material for use in reproduction of sound such as in speaker parts and cones.